This invention relates to a liquid fuel cell, and particularly to a liquid fuel cell capable of stable operation for a prolonged time under controlled supply of fuel and water.
Generally, fuel cells using a liquid fuel are classified into an acid type and an alkali type, and methanol, formalin, hydrazine, etc. are used as fuel. The working principle of such fuel cells will be briefly described, referring to FIG. 1, where numeral 1 shows a fuel cell and symbols + and - show terminals for outputting electricity. The fuel cell 1 comprises a fuel electrode 2-1, an oxidizing agent electrode 2-2 counterposed to the fuel electrode 2-1 (the oxidizing agent electrode can be called "oxygen electrode" when oxygen is used as an oxidizing agent, or "air electrode" when air is used as an oxidizing agent), an electrolyte chamber provided between the oxidizing agent electrode 2-2 and the fuel electrode 2-1, a fuel chamber 4 provided adjacent to the fuel electrode 2-1, and an oxidizing agent chamber 5 provided adjacent to the oxidizing agent electrode 2-2. In FIG. 1, numeral 6 shows the fuel (which may contain water), or a mixture of fuel and electrolyte and also shows its flow direction, and numeral 7 likewise shows the oxidizing agent and also shows its flow direction.
The fuel cell as structured above works as follows. When the fuel 6 is supplied to the fuel chamber 4 and when the oxidizing agent 7 is supplied to the oxidizing chamber 5, the fuel 6 permeates into the fuel electrode 2-1 to generate electrons through the electrochemical reaction. When a load is given to the external circuit, a direct current can be obtained. In this case, a product 81 is formed in the fuel chamber 4. The product is a carbon dioxide gas or carbonate when the fuel is methanol, formic acid or formalin, and nitrogen when the fuel is hydrazine. When the supply of fuel 6 of a circulating type, the product contains excess fuel or electrolyte, and it is necessary to separate and vent the gaseous product from the circulation system.
On the other hand, when the oxidizing agent 7 is supplied to the oxidizing agent chamber 5, the oxidizing agent 7 permeates and diffuses into the oxidizing agent electrode 2-2 to consume electrons through the electrochemical reaction. When the electrolyte is of an acid type, a product 82 is formed. The product is mainly water and contains excess air. When the electrolyte is of a base type, water is formed at the fuel electrode 2-1.
When an aqueous solution of electrolyte such as sulfuric acid or potassium hydroxide is used in the electrolyte chamber 3 in the unit fuel cell 1 structured as in FIG. 1, the aqueous solution leaks from the electrolyte chamber 3 and thoroughly permeates also into the electrodes, and a good cell performance can be obtained. However, the aqueous solution of electrolyte also leaks into the fuel chamber 4 in this case, and thus it is necessary to supply fuel mixture containing the aqueous solution of electrolyte prepared in advance as anolyte. To this end, the fuel chamber 4 is provided with a circulation system for supplying the fuel mixture to the fuel chamber 4 by a pump 9 and a system for supplying the fuel from a fuel tank 10 through a valve 11 to the circulation system, as shown in FIG. 2.
It has been also proposed to use an aqueous solution of polymeric electrolyte in the electrolyte chamber 3 in place of the acid electrolyte, and provide the fuel chamber with a circulation system for a fuel mixture of fuel and water adjusted to a most suitable concentration for the operation as in FIG. 2, and also with a system for supplying the fuel from the fuel tank 10 to the circulation system.
As shown in FIG. 2, the product gas 811 is vented from the circulation system after the passage through the fuel chamber 4, and the remaining mixture 812 is recycled to the fuel chamber.
According to the conventional fuel supply system as described above, a fuel mixture in a constant mixing ratio prepared in advance is supplied to the circulating system from the single fuel tank 10 shown in FIG. 2. However, it has been found that the consumption ratios of fuel and water in the circulation mixture 6 containing the fuel are not always constant, and depend on changes in load, changes in temperature of fuel cell during the operation, even though the load is constant, or changes in flow rate and temprature or humidity of the air supplied as the oxidizing agent.
In a fuel cell using a liquid fuel, the fuel supply system contains two essential components, i.e. fuel and water, and further may contain an electrolyte. In the most cases, these three components, i.e. fuel, water and electrolyte are usually contained in the fuel supply system. Among these three components, it is the fuel and water that are consumed. Usually, it is not necessary to take consumption of electrolyte into consideration. Consumption rate of fuel differs from that of water, because firstly water is always consumed at one electrode, whereas at another electrode water is always formed as a result of the electromotive reaction of a fuel cell, and formation of water at the fuel electrode or the oxidizing agent electrode, depends on the acidity or the alkalinity of electrolyte. That is, in the case of an acidic electrolyte, water is formed at the oxidizing agent electrode and consumed at the fuel electrode, whereas in the case of an alkaline electrolyte, the formation and consumption of water are reversed. In that case, one mole or two moles of water is principally formed with one mole of fuel throughout the reaction, depending on the species of fuel. Since the consumption and formation of water take place at the different electrodes, water actually tends to migrate through the electrolyte chamber to keep a water balance. Even in view of this tendency, water is short at one electrode and in excess at another electrode, owing to much dissipation of water and difficulty to keep the water balance well throughout the electrolyte chamber.
Secondly, the excess or shortage of water due to water imbalance in the water migration between the electrodes largely depends on the operating temperature and the load current.
Thirdly, the excess fuel that is not converted to the electric current at the fuel electrode migrates through the electrolyte chamber and permeates into the oxidizing agent electrode to occasion direct oxidation of the fuel, or water migrates as hydronium ions when the electrolyte ions migrate in the electrolyte chamber in the case of an acidic electrolyte. These phenomena also depend on the load current and operating temperature of a fuel cell. Furthermore, the amount of water carried by the oxidizing agent, for example, air by evaporation at the oxidizing agent electrode side depends on the feed rate, temperature and humidity of the oxidizing agent.
The consumption rate of fuel differs from that of water on the grounds as described above, and thus the supply of a mixture of fuel and water only in a constant mixing ratio from a single tank to the fuel circulation system as shown in FIG. 2 can only meet a change in the amount of only one component among the two components, i.e. fuel and water, in the fuel circulation system including the fuel chamber. That is, adequate control over the fuel and water cannot be made, and stable and prolonged operation of a fuel cell is quite impossible to conduct. That is, the fuel in the fuel circulation system may be so concentrated that the heat is much generated or the current output is lowered, or the supply of fuel fails to catch up with the consumption, so that the fuel becomes short in the fuel circulation system.
In a fuel cell using a liquid fuel, the cell voltage V shows a flat peak in a certain range of concentration C.sub.m of fuel 6 when the current is constant. At a lower fuel concentration C.sub.m, the fuel becomes short and the cell voltage is lowered, whereas at a higher fuel concentration C.sub.m, the excess fuel that fails to take part in the reaction at the fuel electrode 2-1 migrates through the electrolyte chamber 3 and permeates into the oxidizing agent electrode 2-2 to occasion direct combustion of fuel. As a result, the potential on the oxidizing agent electrode 2-2 is lowered with generation of heat, and consequently the cell voltage is lowered. When the fuel concentration is too high or too low (e.g. less than C.sub.m1 or more than C.sub.m2 in FIG. 3), the ratio of the necessary amount of electrical energy-converted fuel to the amount of consumed fuel will be lowered, and thus the fuel ultization efficiency is considerably lowered. Thus, it is very important to select an appropriate fuel concentration.
An appropriate range of the fuel concentration, i.e. the range of fuel concentration, C.sub.m1 to C.sub.m2, shown in FIG. 3, has been so far experimentally studied by many researchers. For example, in the case of an acidic electrolyte type fuel cell using methanol as fuel, it is disclosed in 24th Cell Panel Discussion Lectures No. 2B02, page 254 that the concentration C.sub.m1 is 0.5 moles/l and the concentration C.sub.m2 is 2 moles/l at the current density of 64 mA/cm.sup.2. Japanese Patent Application Kokai (Laid-open) No. 56-118273 discloses that the concentration C.sub.m2 is about 5% by weight (about 1.6 moles/l).
On the other hand, even in a liquid fuel cell using hydrazine as fuel, Japanese Patent Publication No. 48-31300 discloses that stable operation is possible at 1.5% by weight (0.5 moles/l), and if the concentration is less than 1.5% by weight, the voltage is lowered and the temperature is increased.
It is seen from the foregoing that the fuel concentration range for stable operation is about 0.3 moles/l as C.sub.m1 and about 2 moles/l as C.sub.m2.
Thus, the fuel concentration is very important in the fuel cell, and a more accurate apparatus for detecting or measuring the fuel concentration is still required.
A liquid fuel cell provided with an apparatus for detecting a fuel concentration now in practical use is shown in FIG. 4, where the same members as in FIG. 1 and FIG. 2 are indicated with the same reference numerals.
An oxidizing agent 7 is supplied to an oxidizing agent chamber 5 by a blower 111, and discharged as a residual gas 82. On the other hand, a fuel supply system includes a system for circulating a mixture of fuel and an electrolyte solution (the mixture may be called "anolyte") by a pump 9 and a system for supplying an appropriate amount of fuel to an anolyte tank 20 provided in the circulation system from a fuel tank 10 through a valve 17. The circulation system is open to the outside at an appropriate position to discharge the product gas 811.
The fuel is supplied by opening the valve 17, and the opening or closure or control of the valve 17 is made by an apparatus 13 for detecting a fuel concentration provided in the anolyte tank 20 and a valve controller 171.
The apparatus 13 for detecting a fuel concentration comprises an anode electrode 517 (which will be hereinafter referred to merely as "anode"), a cathode electrode 518 counterposed to the anode (the cathode electrode will be hereinafter referred to merely as "cathode"), a power source 519, and a detector 520. The anode 517 comprises a platinum plate 517a and a membrane 517b tightly laid on the platinum plate 517a by pressing.
With such a structure as described above, when a DC voltage of e.g. 0.85 V is applied to between the anode 517 and the cathode 518, the quantity of electric current changes proportionally to the methanol concentration in the anolyte. Thus, it is possible to determine the concentration of methanol as fuel in a very simple structure.
However, the concentration of methanol can be indeed determined by the apparatus with such a structure as described above, but its detection sensitivity is not better, as given below.
Relationship between the fuel concentration and detected electric current is shown in FIG. 12, where curve a shows those determined by an apparatus for detecting a fuel concentration using an anode with the membrane as shown in FIG. 5. The electric current changes with concentration C.sub.m but the change in electric current is small. That is, the detection sensitivity is poor.
Furthermore, the adhesion between the platinum plate 517a and the membrane 517b (FIG. 5) is often inadequate, and the anolyte tends to stay therebetween, deteriorating the response to changes in the methanol concentration. When a platinum-based catalyst layer is laid on the platinum plate 517a in place of the membrane 517b, much detected current can be obtained as shown by curve b in FIG. 12, but there is no change in the detected current in the practical range (about 0.3-about 2 moles/l) and such a structure cannot be used as a sensor.
Cyclic voltammetry using a reference electrode and an apparatus for detecting a fuel concentration by means of a small fuel cell as disclosed in Japanese Patent Application Kokai (Laid-open) No. 56-118273 are also available as another apparatus for detecting a fuel concentration. In the case of the cyclic voltammetry, a reference electrode is required in addition to the detecting electrodes, and also a function generator and other devices are required, complicating the detecting system and deteriorating the reliability, the most important task of the sensor.
In the case of the apparatus using a small fuel cell, not only the apparatus is dipped in the anolyte tank, but also an additional air supply system is required, and there is a difficulty in reduction in the apparatus size as well as in the reliability.
In the case of using methanol or formalin as fuel rather than using hydrazine as fuel, the detected power output changes in a complicated manner even according to the cyclic voltammetry, and the determination is sometimes difficult to make.
There is other procedure for supplying a fuel when an integrated load current becomes constant, since the fuel concentration is proportional to the load current, but when the load is greatly changed or the operation of fuel cell is subject to repetitions of discontinuation, the fuel concentration will be greatly deviated and cannot be practically determined. A gas concentration sensor based on semi-conductors requires much time until it is settled for the measurement, and thus the response becomes poor.
Thus, a liquid fuel cell with a reliable apparatus for detecting a fuel concentration in a simple structure is in keen demand.